Method for the heat treatment of a cable

ABSTRACT

The invention relates to a method in connection with a process for insulating or sheathing conductors or the like, wherein plastic material is extruded into at least one layer on the surface of the conductor (1), and the conductor coated with the plastic material (2, 3, 4) is cooled in a predetermined manner. To avoid problems caused by the shrinking of the plastic material, after the cooling of the conductor (1) and the plastic material in vicinity to it to a predetermined level in the insulating or sheathing process, the surface layer of the plastic material and the plastic material portions in vicinity to it are reheated to a predetermined temperature in a pressurized space filled with a medium.

The invention relates to a method in which plastic material is extrudedinto at least one layer on the surface of a conductor or the like, andthe conductor or the like coated with the plastic material is cooled ina predetermined manner in connection with an insulating or sheathingprocess.

In the production of cables, there occurs problems due to the largechange of the specific volume of plastic materials, such aspolyethylene, on transition from the melting point to the roomtemperature. This is because the large change of the specific volumecauses a disadvantageous shrinking phenomenon to occur in the cable,which tends to shorten the plastic layer around the conductors, thuscausing problems in cable extensions and terminals. The shrinking effecthas previously been reduced by applying a heat treatment to the finishedcable or conductor wound on a reel. A problem therewith is, however,that as the stresses are released, when the cable is bent around thereel, further stress states are created in the plastic material when thecable is straightened. The heat treatment of the cable wound on a reelalso requires plenty of time, at least 2 to 3 days, as well as plenty ofenergy.

The object of the invention is to provide a method by means of which thedisadvantages of the prior art can be eliminated. This is achieved bymeans of a method according to the invention, which is characterized inthat after the cooling of the conductor and the plastic material invicinity to it to a predetermined level in the insulating or sheathingprocess, the surface layer of the plastic material and the plasticmaterial portions in vicinity to it are reheated to a predeterminedtemperature in a pressurized space filled with a medium.

An advantage of the invention is mainly that the above-mentionedshrinking problems can be eliminated in a very short time and by aconsiderably lower consumption of energy as compared with the previouslyused technique. An example of the savings obtained in terms of energyconsumption is that the heat treatment of a specific cable required anamount of energy of about 32 kWh/km when applying the method accordingto the invention. When using the prior art heat treatment with acorresponding cable wound on a reel, the energy consumption is about 46kWh/km if the temperature of the cable is raised from 20° C. to 100° C.In addition, a considerable amount of energy is consumed in heating thereel and in the losses of the heat treatment space. A further advantageof the invention is that, contrary to the prior art technique, no newstress states are created in the cable as the heat treatment accordingto the invention is performed on straight cable in place of cable woundon a reel.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described more closely by meansof an example shown in the attached drawings, wherein

FIG. 1 shows a typical cross-section of an insulated conductor of ahigh-voltage cable;

FIG. 2 is a graphical presentation illustrating variation in differentparameters when the cable conductor enters the cooling step of theprocess;

FIG. 3 illustrates variation in the parameters of FIG. 2 when applyingthe method according to the invention; and

FIG. 4 illustrates the diameters of the plastic material layerscalculated from FIGS. 2 and 3 as compared with the actual diameters whenthe temperature drops to 20° C.

FIG. 1 shows a typical cross-section of an insulated conductor of ahigh-voltage cable. A thin semi-conductive plastic layer, a conductorshielding 2, is provided around the conductor 1. The conductor shielding2 is covered by an insulation 3, and the outermost layer also consistsof a rather thin, semi-conductive corona shielding 4. The finished cablefurther comprises a varying number of different layers, which, however,are well-known to one skilled in the art, and are not as such directlyrelated to the invention. These layers will not be described in greaterdetail herein. The thickness of the plastic layer on the conductor inmedium- and high-voltage cables varies e.g. in the range from 5 to 35mm. A widely used insulation material is cross-linking or thermoplasticpolyethylene.

The layers 2 to 4 on the conductor 1 are extruded on the conductorsimultaneously in a so-called triple extrusion head, into which plasticmaterials for the different layers are usually supplied from threeseparate plastic extruders. When using cross-linking insulation plastic,the maximum extrusion temperature of the plastic should be about 140° C.in order to avoid an untimely cross-linking reaction within the plasticextruder. To reduce the reaction time, the insulation material is heatedafter the plastic extruder typically to a temperature ranging from about190° to 210° C. Heating is usually carried out by means of steam or heatradiation in a pressurized tube. Pressure is required to prevent theformation of bubbles due to the gases formed in the cross-linkingreaction. An excessively rapid surface

Cooling may also contribute to the formation of bubbles within theinsulation layer as it prevents the shrinking of the diameter.

If thermoplastic polyethylene is used as insulation material, theextrusion temperature of the plastic is between 170° and 230° C. As noreactions occur in the plastic material, pressure as such is not neededin this case, whereas the cooling effected by air or warm water musttake place very slowly in order to avoid the formation of bubbles due tothe shrinking of the plastic material. For this reason, a cableinsulated by a thermoplastic material is also cooled under pressure,i.e. at a pressure above the atmospheric pressure.

Today it is customary to use a low-density polyethylene cross-linkableby peroxide as insulation material in most medium- and high-voltagecables, and the manufacturing process generally employs a so-calleddry-vulcanization method, wherefore the method according to theinvention will be described below with reference to the above-mentionedmaterial and the vulcanization principle. Dry-vulcanization means thatthe cross-linking of the insulation plastic takes place in completelydry conditions in a pressurized shielding gas. The dry shielding gas maybe e.g. nitrogen. Even though the following description relies on theabove-mentioned matters, it is, however, to be noted that otherprinciples and materials are not excluded from the scope of protection.

In dry vulcanization, the insulated conductor shown in FIG. 1 typicallyemerges from the triple extrusion head into a heated tube which isusually pressurized by nitrogen so as to heat the plastic layers,typically by heat radiation, to a temperature clearly above theextrusion temperature of the plastic in order to accelerate thecross-linking reaction. Run parameters such as the speed of theconductor and the temperature profile of the cross-linking section ofthe heated tube are calculated by a computer.

FIG. 2 is a graphical presentation showing the temperature 5 of theconductor with the plastic layers on emerging from the cross-linkingsection to a pressurized water cooling step at 20° C., the temperature 6of the centre point of the insulation, and the temperature 7 and theouter dimension 8 of the surface layer as a function of time. Thediameter of the insulated conductor is 30 mm at 20° C., and the diameterof the conductor 1 itself is 10 mm. These values are obtained by acomputer program which divides the plastic into 10 cylinder-symmetricallayers and calculates the transfer of heat between the layers and theheat expansion of the layers.

In the specific case described above, the specific volume of the plasticdecreases about 13% when the temperature drops from the melting point,about 110° C., to 20° C. As it may be assumed that the metal conductoris fully rigid as compared with the plastic, the entire volume changehas to take place in the cross-section, that is, in the radialdirection.

FIG. 2 thus illustrates a way of cooling used in connection with theconductor insulation process, in which the insulated conductor is cooledin a predetermined manner.

In the invention, the cooling is not carried out in the above-describedway, but it is essential that after the conductor 1 and the plasticmaterial in vicinity to it have cooled to a predetermined level in theinsulation process, the surface layer of the plastic material and theplastic material portions in vicinity to it are reheated to apredetermined temperature. In a preferred embodiment, the surface layerof the plastic material and the material portions in vicinity to it arereheated to a temperature slightly below the melting point of theplastic material after the conductor 1 and the plastic material invicinity to it have cooled substantially to the melting point of theplastic material.

FIG. 3 shows curves corresponding to the curves shown in FIG. 2 andobtained when using the method according to the invention. FIG. 3 showsthe temperature 9 of the conductor, the temperature 10 of the centrepoint of the insulation, and the temperature 11 and the outer dimension12 of the surface layer as a function of time.

As appears from FIG. 3, the heating of the conductor with 140° C. waterhas been started when the temperature of the conductor has dropped to117° C. Heating has been continued until the temperature of the centrepoint of the insulation has risen to 100° C., whereafter the cooling ofthe conductor with 20° C. water has been restarted. The curves shown inFIGS. 2 and 3 are identical for about 7 minutes, as the conditions areidentical. It can be seen from the curves shown in FIG. 3 that thetemperature 11 of the outermost layer rises rapidly after 7 minutes toabout 140° C., the temperature 10 of the centre point starts to rise,the decrease in the temperature 9 of the conductor becomes slower, andthe diameter 12 increases. At about 9.1 minutes, it can be seen that thetemperature of the plastic from the conductor to the centre point isabout 100° C., and from the centre point to the outer layer >100° C. Ifthe temperature of 100° C. is assumed to be a kind of mechanicalhardening temperature for this kind of plastic, i.e. that the plasticbehaves below this temperature similarly as solid substances, such asmetal, with respect to thermal expansion and mechanical stresses, itappears from the diameter curve 12 of FIG. 3 that the outer layer ishardened to a diameter of 31.7 mm. From FIG. 2, in turn, it appears thatthe temperature 7 of the outer layer ultimately falls below 100° C. whenthe outer diameter 8 is 33.0 mm.

The computer program divides the plastic insulation of the conductor at20° C. into ten layers of uniform thickness. The program also stores theinner and outer diameter of each layer when the temperature of the layerultimately falls below 100° C. The dimensions of all layers would, ofcourse, change in the same proportion when the temperature of the layersdecreases from 100° C. to 20° C., if the layers were apart from eachother. As this is not the case and as, for instance, the diameter of theoutermost layer is 30 mm irrespective of whether the conductor is cooledin accordance with FIG. 2 or 3, the outer layer has to contract from33.0 mm to 30 mm when applying cooling in accordance with FIG. 2 andfrom 31.7 to 30 mm when cooling in accordance with FIG. 3 as thetemperature drops from 100° C. to 20° C.

If the diameters of layers taken apart from each other are compared withthe actual diameters when the temperature drops to 20° C., thepercentage curves shown in FIG. 4 are obtained. The curve 13 is obtainedby cooling in accordance with FIG. 2, and the curve 14 correspondinglyby cooling according to the invention, illustrated in FIG. 3. The curve13 shows that when taken apart the outermost layer would remain about4.6% oversize, so that there is actually tangential compression stresspresent in it, which extends up to the layer 5, and is then changed intotensile stress. The outer layer of the conductor cooled by means of themethod according to the invention remains only about 0.1% oversize, andthe tangential force consists of tension almost throughout the thicknessof the plastic layer. It is obvious that the innermost layer behavessimilarly in both cases, as the conductor 1, which is assumed to beunchanged, lies below it. As the tangential tensile stress present inthe layers increases the surface pressure exerted on the conductor, thecooling in accordance with FIG. 3 provides a greater friction forcebetween the plastic and the conductor than the cooling according to FIG.2. Consequently, the cooling according to the invention decreases thelongitudinal shrinkage.

The method according to the invention has a wide range of differentapplications. One example of such applications is the apparatusimplementation described in Finnish Patent Specification 52299 and thecorresponding U.S. Pat. No. 4,035,129, in which a tube defines a spacethrough which the conductor passes. The tube also serves as a heatingresistor in the apparatus. The heat treatment of the cable may berealized by dividing even the cooling section of the tube into separatezones with separate gas or water circulations. If the cooling medium iswater, that is, water cooling and, in the heat treatment section, waterheating are used, the tube heats the water and the water heats theconductor. When gas is used, the heating of the conductor takes placemainly by heat radiation. In both cases, it is, of course, necessary toprevent the mixing of the cooling medium between the heat treatmentsection and the cooling zones on its both sides.

The above-described embodiment is by no means intended to restrict theinvention, but the invention may be modified within the scope of theclaims as desired. Accordingly, it is obvious that the invention is notin any way restricted to a cross-linking reaction, but it may be appliedin other connections as well. For example, stress states may occur inthe relatively thick sheathing layer in certain cable types due toexcessively rapid cooling, which stresses can be removed in anadvantageous manner by heat treatment. The invention is no% eitherrestricted to medium- and high-voltage cables, but it can be appliedmore widely, e.g. in the sheathing of optical cables.

I claim:
 1. In a method for insulating or sheathing conductors, whereincross-linkable plastic material is extruded onto an outer surface of aconductor, and thereafter cooled in a predetermined manner, animprovement comprising:a) cooling the conductor and proximatecross-linkable plastic material substantially to the melting point ofthe cross-linkable plastic material; and then b) reheating the surfaceof the cross-linkable plastic material and proximate cross-linkableplastic material substantially to the melting point of thecross-linkable plastic material in a pressurized space filled with gas.